Electrolytic cell and process for metal reduction

ABSTRACT

An improved electrolytic cell and process are provided wherein metals and metal alloys are formed from oxides or nitrides in a molten salt, without the evolution of halogen or halogen compounds, with less corrosion and reduced power consumption by the use of an electrode having an extended or substantially increased surface area effective for the evolution of oxygen and carbon oxide, and a molten salt electrolyte effective at low temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

By way of international application PCT/US88/04565, this is acontinuation-in-part of now-abandoned U.S. patent application Nos.07/138,391 filed Dec. 28, 1987 and 07/197,889 filed May 24, 1988, nowabandoned.

DESCRIPTION

1. Technical Field

This invention relates to an electrolytic cell and to the electrolyticformation of an electrode product using a molten salt.

2. Background of Invention

In the electrolytic decomposition of alumina in molten salts, theHall-Heroult process is commonly employed. Examples of Hall-Heroultcells are shown in U.S. Pat. Nos. 3,839,167, 3,996,117, and 4,269,673.

It has been known that the power consumption can be decreased in aHall-Heroult cell by placing the anode and cathode in close proximity toone another. However, to maintain typical operating temperatures andgain the energy conserved in a Hall-Heroult cell as a result of reducinganode-cathode distance an equal reduction of thermal losses to theenvironment is required. The degree of insulation possible to reducethermal energy lost is limited by the need to maintain a frozenelectrolyte layer on the sidewall for protecting the lining materials.An increase in current could be used to achieve normal operatingtemperature, but this is limited by the magnetic stability of the celland would reduced the energy conserved.

DISCLOSURE OF INVENTION

This invention enables the use of molten salts with limited reactant,e.g. alumina, solubility to be such as aluminum. An advantage is thatsome of these salts have low liquidus temperatures, e.g. in the range300° to 900° C., preferably in the 500° to 800° C., such that the moltensalt operating temperature can be lower than usual Hall-Heroult celloperating temperatures, for instance from 900° C. down to the meltingpoint of aluminum (or even below the melting point of aluminum if it isdesired to produce solid aluminum). These salts are not as corrosive asthose conventionally used in the Hall-Heroult process, are lower indensity, and have lower alkali metal activity. The reduced corrosioneliminates the need for frozen electrolyte to protect the materials usedin cell construction. The lower density of the salt improves cellstability because of the greater density difference between the metaland the salt, requiring greater force differences to create the sameamplitude waves at the bath-metal interface. The lower alkali metalactivity improves current efficiency, and eliminates swelling of carbonmaterials in the cell, enhancing cell life. The invention additionallyprovides the potential for the use of graphitic cathodic floors, ratherthan the usual carbon floors; graphitic floors are currently notfeasible, because of high intercalation of alkali metal species intothem, leading to premature failure.

Thus, it is proposed that improved results can be achieved with the useof an electrolytic cell, e.g. a Hall-Heroult cell, comprising anelectrode having an evolution of the desired electrode product, and amolten salt with limited reactant solubility. According to theinvention, reactant solubility is ≦1 wt %. This allows the use of allbroader ranges of molten salt, at lower operating temperatures withbenefits in both physical properties and lessened chemical reactivity ofthe molten salt with cell components such as the materials ofconstruction (for instance the refractories used for the sidewalls andthe floor) and the electrodes.

In a preferred embodiment, the anode and cathode are in close proximityto one another, i.e. 0.25"-1.25", and the outside walls of the cell arethermally insulated sufficiently to maintain the electrolyte temperatureat the decreased power levels. Operation can be without a frozensidewall; some suspended solid reactant will usually be present.

Percentages herein are on a weight basis, unless indicated otherwise.Reactant concentrations are based on total weight of molten salt plusreactant, although such is not an essential point in view of the lowreactant concentrations.

The invention comprises an improvement concerning the electrolyticdecomposition of a substance in a molten salt electrolyte, e.g., achloride and/or fluoride electrolyte, which typically has low solubilityfor an oxide whose decomposition is desired. According to the invention,an electrode is employed having an extended or substantially increasedsurface area effective for the electrolysis of the desired reactant,e.g. alumina, and the evolution of a desired electrode product, such asoxygen and/or carbon oxides rather than halogen or halogen compounds,such as C_(x) F_(y) (e.g. CF₄).

The use of an extended surface area anode, for instance, results inselective electrolysis of a metal oxide at low concentrations in anelectrolyte. For example, the use of an anode with a plurality of holesor channels to increase the surface area was found to decompose aluminumoxide (alumina) in preference to chloride electrolyte in which it wascontained. The smaller the hole size the larger the extended surfacearea can become, improving the effectiveness of the electrode. However,it is necessary for the species of interest to gain access to the depthof the electrode by electrochemical migration and/or convection. Thesize of the hole channel for gas evolving electrodes should be largeenough to avoid gas bubbles blocking the electrical flow of current orproviding a path of high resistance. It is desirable to circulate theelectrolyte to provide a means of suspending solid reactant andimproving its dissolution. Our means of accomplishing this is through anappropriately designed gas and/or magnetically induced electrolyte flowfield within the cell. A means of achieving this is to allow the gas tomove upwardly through holes or channels, in order to use the buoyancy ofthe gas to pump the electrolyte. This action is promoted by providing areturn channel to create a flow loop. Therefore, adequate hole size andflow control geometry should be provided to achieve this effect. U.S.Pat. No. 3,822,195 utilizes gas-lift pumping to circulate molten salt.The gas-lift pumping can be coupled with magnetically driven flow toenhance the overall effectiveness.

An important feature of the invention is that despite low reactantsolubility nevertheless appreciable current densities are achieved. Withrespect to current density at the anode(s), superficial anode currentdensities greater than 1, 2, 3, 4, and even 5 or 6 amperes/square inch(0.15, 0.3, 0.45, 0.6,and even 0.75 or 0.9 amperes/square centimeter)are achieved. Superficial anode current density is determined bydividing the cell current by the cross sectional area of the bottom ofthe anode assuming there are no holes or channels in it. This area isreferred to herein as the "superficial area".

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a laboratory electrolytic cell.

FIG. 2 is a cross sectional view along the cutting plane II--II of FIG.1.

FIG. 3 is an end view of half of a production cell with a verticalcutting plane through an anode for showing internal structure of theanode.

MODES FOR CARRYING OUT THE INVENTION

Referring now to FIGS. 1 and 2 of the drawing, 1 is a cylindrical anode,2 is one of the channels in the anode, 3 is molten salt, and 5 is amolten metal cathode. The perimeter of the anode, for test purposes, isshielded with a non-conductor 4 to prevent this area from taking part inthe electrochemical reaction. The anode is suspended in a quartz vessel6, and 7 is a graphite liner for the cathode. Gas bubbles 8 are shownrising from the channels 2. Depending on the densities of the moltensalt and molten metal, the anode and cathode may be reversed. FIG. 2shows the end view of the anode illustrating a typical hole patterndrilled into the anode to extend its surface area. A toroidally shapedcirculation pattern is set up in the molten salt due to the gas-liftaction of the bubbles 8 rising in the channels 2, with the salt risingin the channels and then falling down the outer sides of non-conductor4, thence to sweep across the upper surface of the cathode 5, and againup through the channels. This circulation acts to suspend undissolvedalumina particles and to incorporate into the molten salt thereplenishment alumina particles as such is fed from the top of the cellinto the molten salt.

FIG. 3 shows the half of a production cell left of centerline 24, where11 is an anode, 12 is one of the channels in the anode, 13 is the moltensalt bath, and 14 is a carbonaceous, electrically conductive floor.Molten metal (e.g. aluminum) cathode 25 rests on floor 14. Insulation isprovided by bottom lining 15, sidewall 16 and lid 17,18. Rod 19 is ananode collector bar for providing d.c. electrical current to the anode11. The cell lid is attached to a superstructure 21 via elbow 20 andrests on the sidewall 16. Current is removed from the cell throughcathode collector bar 22. Sleeve 23 protects the connection between theanode collector bar and the anode from molten salt. A larger anode canbe employed, because there is no frozen electrolyte to interfere withits positioning. Depending on the relative densities of the molten saltand molten metal, the anode and cathode may be reversed. The circulationpattern executed by the molten salt in the cell of FIG. 3 will beinfluenced both by the gas-lift action of the evolved anode product andby electromagnetic phenomena, and the resulting circulation patternexecuted by the molten salt will be the result of those combinedeffects. Electromagnetic effects become more important in productioncells because of their large size (e.g. 15-foot by 40-foot rectangulardimensions in the horizontal plane) and the larger electrical currentpassing through them (e.g. 125 to 150 kiloamperes). For furtherinformation on circulation patterns caused by "Hydrodynamic Modeling ofCommercial Hall-Heroult Cells" appearing in "Light Metals 1987", pp269+. The circulation will again act to keep undissolved aluminaparticles in suspension. Points of addition of replenishment alumina maybe chosen based on the molten salt circulation pattern to effect anoptimum, rapid incorporation of fed alumina into the molten salt.

The anode configuration is one in which the solid phase is continuousand of relatively high conductivity compared to that of the electrolyte.Consumable anodes and inert anodes may be used. Consumable anodes aremade of carbon and react to form carbon dioxide and carbon monoxide, therelative amounts, as is known, being indicative of the currentefficiency. An example of inert anode is set forth in U.S. Pat. No.4,620,905. In 4,620,905, a cermet is provided in which nickel is presentas a continuous phase of relatively high conductivity as compared to theceramic phase. The characteristic feature of inert anodes is that theyare not consumed during the electrolysis, so that, in the electrolysisof alumina (Al₂ O₃), oxygen is evolved as the anode product, rather thancarbon oxides as is the case when using carbon anodes.

Suitable molten salt compositions are those which have a limitedsolubility for alumina. Examples include: about 5 to about 100% metalchlorides (i.e. alkali and alkaline earth metal chlorides, and Group IIImetal chlorides, e.g., sodium and potassium chlorides, magnesium andcalcium chlorides, aluminum chloride, etc.), and about 2 to about 100%metal fluorides (i.e. alkali and alkaline earth metal fluorides, andGroup III metal fluorides, e.g., sodium and potassium fluorides,magnesium and calcium fluorides, aluminum fluoride, etc.). The chloridesare, in general, less chemically aggressive than the fluorides. Anexample of a chloride-based molten salt comprises about 0.5 to about 15wt. % aluminum chloride, from about 3 to about 40 wt. % of an alkalineearth metal chloride selected from the group consisting of barium,calcium, magnesium, and strontium chloride, from about 10 to 90% lithiumchloride and about 10 to 80 wt. % sodium chloride and has a NaCl/LiClratio of about 2.33. It is molten at less than about 650° C. This bathis the subject of U.S. Pat. No. 4,440,610.

Suitable fluorides are cryolite (NA₃ AlF₆), MgF₂, AlF₃, potassiumfluoride and calcium fluoride. An example of a fluoride-based moltensalt is formed from about 35 wt. % lithium fluoride, about 45 wt. %magnesium fluoride and about 20 wt. % calcium fluoride. The lowtemperature operation made possible by this molten salt is indicated bythe fact that it has a solidus temperature of approximately 680° C. Theweight ratio of NaCl/LiCl or NaCl/KCl is preferably between about 0.25or 4.

The reactant, e.g. alumina, in the molten salt can be present at aconcentration of about 0.1 to about 2%, part of which can be present asundissolved, solid suspension.

Mixtures of chlorides and fluorides may be advantageous, in order toachieve desires physical properties (density, viscosity, etc.) andchemical reactivity.

While the above is illustrative, a number of other anodes and baths maybe used. The following examples are preferred embodiments. All parts areby weight unless otherwise specified, as elsewhere in the specificationand claims.

EXAMPLE 1

The apparatus used in this example is shown in FIG. 1, except that ananode of rectangular cross section was used. The apparatus was heated byelectrical resistance to bring the chloride-base electrolyte in a quartzcrucible to a temperature of 740° C. The nominal electrolyte compositionwas 64 wt. % NaCl, 27 wt. % LiCl, 4 wt. % AlCl₃, 5 wt. % AlF₃, and 2 wt.% Al₂ O₃. The alumina in this electrolyte had an estimated solubilityless than 0.2 wt. %. The sides of the anode were shielded with boronnitride to eliminate the sides of the anode as electrolysis regions.

In this example, a superficial anode current density of 2.0 amperes/sq.in. was achieved with no measurable chlorine generation (<1.0 ppm). Acurrent efficiency of aluminum production of greater than 80% wasmeasured by CO₂ and CO evolution. In this system the carbon anode wasprepared by drilling fifty-two, 0.375 in. diameter holes through the sixinch length of the anode. The bottom cross section of the anode was 4.5in. by 3 in., or 13.5 sq. in. The extended area of the anode generatedby the holes through the entire length was 360 sq. in. A portion of thisextended area will be available for electrolysis depending on themagnitude of the overpotential associated with the electrochemicalreaction. In the case of this example, a measure of the change in holesize after electrolysis demonstrated that the hole diameter hadincreased three inches into the depth of the electrode illustratingelectrochemical activity to this depth.

EXAMPLE 2

Using an apparatus as described in Example 1, electrolysis was carriedout in an electrolyte composed of 43 wt. % NaCl, 43 wt. % KCl, 12.5 wt.% cryolite and 1.5 wt. % alumina. The estimated alumina solubility wasless than 0.6 wt. %. The anode had a 55/8 in. diameter, was 6 in. longand made of carbon. The perimeter area was electrically insulated withboron nitride as in the previous example. The anode was drilled throughits length with one hundred two, 0.375 in. diameter holes. This providedan extended surface area of 721 sq. in. This is approximately 29 timesthe superficial area of this anode. A superficial current density of 4.2amperes/sq. in. was achieved with no measurable chlorine generation.Aluminum production current efficiency of greater than 80% was measuredby CO₂ --CO evolution.

EXAMPLE 3

The apparatus described in Example 1 was employed and the electrolysisof MgO was carried out in an electrolyte composed of 69.25 wt. % LiCl,25 wt. % KCl, 5 wt. % LiF and 0.25% MgO. The electrolysis of MgO wascarried out with no measurable chlorine generation at a superficialcurrent density of 2.2 amperes/sq. in. An aluminum pool was used as thecathode to keep the magnesium from floating. Analysis of the aluminumpool at the completion of this experiment resulted in a content of 2.69wt. % magnesium.

EXAMPLE 4

An extended surface area anode is used in this process the basic designof which is shown in FIGS. 1 and 2. The concept as applied to commercialaluminum production is shown in FIG. 3, where, however, an anode ofrectangular cross section is used. A thermal balance is achieved basedon the electrical energy used, the aluminum produced, heat losses to theenvironment, heat of reaction, and an operating temperature of 750° C.The nominal electrolyte composition is 42.75% NaCl, 42.75% KCl, 12.5%Na₃ AlF₆, and 2% Al₂ O₃. The alumina in this electrolyte has anestimated solubility less than 0.5%.

In this example, a superficial anode current density of 5/875amperes/sq. in. at a cell voltage of about 3.22 V is estimated with noexpected chlorine generation due to the extended surface area of theanode. The anode-cathode distance is 1.00 in. A current efficiency ofaluminum production of about 92% or more is expected. In this system thecarbon anode is prepared by drilling 0.375 in. diameter holes throughthe 15 in height of the anode to achieve a porosity of 30-40%. Thebottom cross section of the anode is 21 in. by 39.375 in., or 827 sq.in. The extended area of the anode generated by the holes through theentire height is 46,312 sq. in. A portion of this extended area will beavailable for electrolysis depending on the magnitude of overpotentialassociated with the electrochemical reaction. Typically, 1 to 3 inchesof the hole depth facing the cathode is electrochemically active.Therefore, increases of 4 to 12 times in surface area for electrolysisare expected. The anode top is submerged below the upper surface of theelectrolyte to aid in electrolyte circulation. The connection of theanode rod to the carbon is protected from the electrolyte by a sleeve toprotect the salt from getting to the junction between the anodecollector bar and the carbon anode. The cell design shown in FIG. 3takes advantage of the following attributes of the invention: lowertemperatures, low reactant-solubility, lower corrosion, greater densitydifference metal to salt, lower alkali metal activity and higherelectrical conductivity. Expected energy required to produce aluminum,based on an energy balance calculation using the 3.22 volts and 92%current efficiency cited above, is only about 4.7 Kwh/lb (kilowatt-hoursper pound of aluminum) compared to conventional rates of about 6 ormore.

In preferred embodiments, the extended surface area of the electrode isat least about 2 times and more preferably, at least about 15 times thatof the superficial area of the electrode. The electrode can beconsumable or inert.

We claim:
 1. A metal reduction process of electrochemically producingelectrode product from metal oxide during electrolysis in a molten saltcomposition, comprising providing an anode having an extended surfacearea for the selective evolution of gaseous electrode product, the metaloxide having a solubility <1 wt. % in the molten salt composition. 2.The process of claim 1 wherein the gaseous product comprises oxygen orcarbon oxide.
 3. The process of claim 1 wherein the anode comprises aplurality of interior holes to increase the surface area of the anode.4. The process of claim 3 wherein the holes create a 15 to 75% voidvolume in the anode and extend through the anode.
 5. The process ofclaim 3 wherein the holes are dimensioned to provide gas-lift pumping ofthe molten salt composition through the channels.
 6. The process ofclaim 1 wherein the extended surface area is at least about 2 times thatof the superficial area of the anode.
 7. The process of claim 1 whereinthe extended surface area is at least about 30 times that of thesuperficial area of the anode.
 8. The process of claim 1 wherein thesalt composition comprises metal chlorides or metal fluorides, or both,in the amount of from about 0 to about 100% metal chlorides, 0 to about100% metal fluorides, and having a liquidus temperature less than about900° C., and the electrolysis comprises deposition of molten aluminumand evolution of oxygen or carbon oxide as electrode product.
 9. Theprocess of claim 1 wherein the salt composition comprises lithiumfluoride, magnesium fluoride and calcium fluoride which is molten atless than about 850° C.
 10. The process of claim 1 wherein the saltcomposition comprises sodium chloride, potassium chloride, and cryolite.11. The process of claim 1 wherein the salt composition comprises about42.75% sodium chloride, about 42.75% potassium chloride, and about 12.5%cryolite.
 12. The process of claim 1 wherein the anode comprises carbonwhich is consumed during the electrolysis.
 13. The process of claim 1wherein the anode faces a cathode comprising molten metal.
 14. Theprocess of claim 1 wherein the electrode product further comprises ametal or metal alloy.
 15. The process of claim 1 wherein the anode isconsumed during the electrolysis.
 16. The process of claim 1 wherein theanode is inert during the electrolysis.
 17. A process as claimed inclaim 16 wherein said inert anode comprises a cermet material.
 18. Theprocess of claim 1 wherein the salt composition contains a fluoride. 19.The process of claim 18 wherein the fluoride comprises cryolite,potassium fluoride or calcium fluoride.
 20. The process of claim 1wherein temperature at a sidewall-molten salt interface is above theliquidus temperature of the molten salt composition, whereby there is nofrozen sidewall.
 21. The process of claim 1 wherein alumina is presentas metal oxide in a concentration range 0.1 to 2%.
 22. The process ofclaim 1 wherein the molten salt composition executes a circulationpattern for suspending and circulating undissolved particles of metaloxide.
 23. The process of claim 1 wherein a molten metal cathode restson a floor of graphitic carbon and the molten metal cathode containsalkali metal constituents.
 24. The process of claim 1 whereinsuperficial anode current density is greater than 1 ampere/square inch.25. The process of claim 24 wherein superficial anode current density isgreater than 2 amperes/square inch.
 26. The process of claim 25 whereinsuperficial anode current density is greater than 3 amperes/square inch.27. The process of claim 26 wherein superficial anode current density isgreater than 4 amperes/square inch.
 28. The process of claim 27 whereinsuperficial anode current density is greater than 5 amperes/square inch.29. A metal reduction process of electrochemically producing oxygen orcarbon oxide anode product from alumina during electrolysis in a moltensalt composition, comprising providing an anode having an extendedsurface area for the selective evolution of such product, the aluminahaving a solubility <1 wt. % in the molten salt composition.
 30. A metalreduction process of electrochemically producing electrode product frommetal oxide during electrolysis in a molten salt composition, comprisingproviding an anode having an extended surface area for the selectiveevolution of a gaseous electrode product, the anode comprises carbonanode, said metal oxide has a concentrated ≦2 wt. %.
 31. A process asclaimed in claim 30 wherein said carbon anode is consumed during theelectrolysis.
 32. The process of claim 30 wherein the gaseous productcomprises oxygen or carbon oxide.
 33. The process of claim 30 whereinthe anode has a plurality of interior holes to increase the surface areaof the anode.
 34. The process of claim 33 wherein the holes aredimensioned to provide gas-lift pumping of the molten salt compositionthrough the holes.
 35. The process of claim 33 wherein the holes createa 15 to 75% void volume in the anode and extend through the anode. 36.The process of claim 30 wherein the extended surface area is at leastabout 2 times that of the superficial area of the anode.
 37. The processof claim 30 wherein the extended surface area is at least about 30 timesthat of the superficial area of the anode.
 38. The process of claim 30wherein the salt composition comprises metal chlorides or metalfluorides, or both, in the amount of from about 0 to about 100% metalchlorides, 0 to about 100% metal fluorides, and having a liquidustemperature less than about 900° C., and the electrolysis comprisesdeposition of molten aluminum and evolution of oxygen or carbon oxide aselectrode products.
 39. The process of claim 30 wherein the saltcomposition comprises lithium fluoride, magnesium fluoride and calciumfluoride which is molten at less than about 850° C.
 40. The process ofclaim 30 wherein the salt composition comprises sodium chloride,potassium chloride, and cryolite.
 41. The process of claim 30 whereinthe salt composition comprises about 42.75% sodium chloride, about42.75% potassium chloride, and about 12.5% cryolite.
 42. The process ofclaim 30 wherein the anode faces a cathode comprising molten metal. 43.The process of claim 30 wherein electrode product further comprises ametal or metal alloy.
 44. The process of claim 30 wherein the anode isconsumed during the electrolysis.
 45. The process of claim 30 whereinthe salt composition contains a fluoride.
 46. The process of claim 45wherein the fluoride comprises cryolite, potassium fluoride or calciumfluoride.
 47. The process of claim 30 wherein temperature at asidewall-molten salt interface is above the liquidus temperature of themolten salt composition, whereby there is no frozen sidewall.
 48. Theprocess of claim 30 wherein alumina is present as metal oxide in aconcentration range 0.1 to 2%.
 49. The process of claim 30 wherein themolten salt composition executes a circulation pattern for suspendingand circulating undissolved particles of metal oxide.
 50. The process ofclaim 30 wherein a molten metal cathode rests on a floor of graphiticcarbon and the molten metal cathode contains alkali metal constituents.51. The process of claim 30 wherein superficial anode current density isgreater than 1 ampere/square inch.
 52. The process of claim 51 whereinsuperficial anode current density is greater than 2 amperes/square inch.53. The process of claim 52 wherein superficial anode current density isgreater than 3 amperes/square inch.
 54. The process of claim 53 whereinsuperficial anode current density is greater than 4 amperes/square inch.55. The process of claim 54 wherein superficial anode current density isgreater than 5 amperes/square inch.
 56. A metal reduction process ofelectrochemically producing electrode product from a metal oxide duringelectrolysis in a molten salt composition, comprising providing a carbonanode spaced in a chloride-containing molten salt from a cathode, saidmetal oxide having a solubility <1 wt. % in said molten saltcomposition; the carbon anode having, relative to the cathode, anextended surface area for the evolution of carbon oxide anode product.